drm/i915/gvt: update vgpu workload head pointer correctly

drm/i915/gvt: update vgpu workload head pointer correctly

when creating a vGPU workload, the guest context head pointer should
be updated correctly by comparing with the exsiting workload in the
guest worklod queue including the current running context.

in some situation, there is a running context A and then received 2 new
vGPU workload context B and A. in the new workload context A, it's head
pointer should be updated with the running context A's tail.

v2: walk through guest workload list in backward way.

Cc: stable@vger.kernel.org
Signed-off-by: Xiaolin Zhang <xiaolin.zhang@intel.com>
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>